1. Field of the Invention
This invention relates to transistor switching circuits. In particular it relates to an improved switching circuit of the current switch type.
2. Description of the Prior Art
The performance of digital computers and other systems employing switching circuits are dependent upon the switching speed of the individual circuits. In general, the higher the switching speed of a circuit, the higher may be the speed of the system.
The current switch disclosed by H. S. Yourke in U.S. Pat. No. 2,964,652 is superior to all other comparable commercially-available switching circuits with respect to switching speed. It is well known, for example, that the current switch circuit is around five to ten times faster than comparable bipolar resistor-transistor-logic (RTL), diode-transistor-logic (DTL) and the transistor-transistor logic (TTL) circuit families. Moreover, it is far superior to any circuit family using field effect transistors.
In addition to high speed operation, the current switch offers excellent DC stability and good noise immunity.
For these reasons, the current switch logic family is used quite extensively in high speed digital circuitry and high performance systems requiring digital components. However, the current switch suffers from certain disadvantages as compared, say, to TTL and DTL. In particular, the power dissipated by the individual logic circuit is greater for the current switch than for the other named circuit families, primarily because one portion of the circuit or the other is always conductive. Current relates to power which must be dissipated as heat.
At the present time, in fact, the number of current switch circuits which can be "packed" on a semiconductor chip of a given size is limited only by the power dissipated within the chip. Above a certain level, around 5 watts, it is virtually impossible to dissipate sufficient heat to avoid destroying the circuits on the chip completely. At the present time the art is limited to accommodating around 600 current switch circuits on a chip around 200 mils square. It is physically possible to place more circuits on the chip from the standpoint of semiconductor area, surface wiring and input/output pads and pins. However, the circuit quantity is power-limited.
Thus, in general, it is possible to place more TTL or DTL logic circuits on a semiconductor chip than current switch circuits. For well-known reasons of reducing manufacturing costs and increasing reliability, the art is continually attempting to accommodate a maximum number of circuit components on a single semiconductor chip.
The interaction between circuit speed and power dissipation has directed the attention of circuit designers in recent years to the concept of "speed-power product", which is simply the multiplication of average circuit switching speed and the power dissipated by the circuit. For example, a circuit switching at 10 nanoseconds and dissipating 5 milliwatts of power (heat) has a speed-power product of 50 picojoules. At the present time, the speed-power product of advanced TTL circuits is practically equal to that of standard current switch circuits.
One reason for the relatively high power required by present current switching circuits is the need for load resistors through which all current flows. As a result, the resistors dissipate a good deal of heat (as well as requiring a great amount of area as compared to transistors or diodes).